A conventional sewing machine disclosed in JP49-110450A includes an upper shaft driving a needle bar and a lower shaft driving a looptaker. The upper shaft is connected to a device for controlling a zigzag width for shifting the needle bar arm laterally. An apparatus for adjusting a timing of a needle and a looptaker is provided between the lower shaft and the looptaker (a looptaker shaft). The lower shaft and the looptaker shaft are connected by means of a pulley and a belt so that a rotation of the lower shaft is transmitted to the looptaker shaft via the belt. The belt contacts four idlers, i.e., two idler units. The sewing machine disclosed in JP49-110450A includes two idler units. The idler unit is connected to the device for controlling the zigzag width via a simple gear mechanism employing helical gears. A driving force is transmitted from the device for controlling the zigzag width to the idler units via the gear mechanism so that the two idler units operate at the same time in cooperation with the device for controlling the zigzag width. Belt tension changes as the idler units operate, thereby transmitting a non-uniform rotation to the looptaker shaft.
However, with the above-described sewing machine, the tension of the belt connecting the lower shaft and the looptaker shaft assumes inconstant because the two idler units operate at the same time by means of the simple gear mechanism employing the helical gears.
The inconstant belt tension, such as being high or low, may cause a torque difference between the upper shaft and the lower shaft and/or, noise and vibration.
In addition, unexpected changes occur to a rotation speed of the looptaker. Due to this, the changes of the rotation speed of the looptaker fails to follow a lateral shifting movement of the needle bar arm (i.e., the needle bar and a needle), thus causing a lag in the timing of the needle and the looptaker, that is, the timing when a hook of the looptaker encounters the needle. This timing lag may cause skipped stitches.
Furthermore, a loosen belt causes “tooth jumping” which may lead to a lag of a preset reference timing between the lower shaft and the looptaker shaft. This lag also may cause skipped stitches.
As described above, when the belt tension is inconstant relative to the rotation speed of the looptaker, a stable operation of a sewing machine may not be assured.
A need thus exists for an apparatus for adjusting a timing of a needle and a looptaker of a sewing machine, which is not susceptible to the drawback mentioned above.